1. Field of the Invention
The present invention relates to a color cathode ray tube and more specifically to a color cathode ray tube in which mechanical stress due to internal pressure made by evacuation is decreased.
2. Description of the Background Art
FIG. 1 shows a schematic diagram illustrating the structure of a general color cathode ray tube. As shown in FIG. 1, the color cathode ray tube generally includes a glass envelope having a shape of bulb and being comprised of a faceplate panel 1, a tubular neck 13, and a funnel 2 connecting the panel 1 and the neck 13.
The panel 1 comprises faceplate portion and peripheral sidewall portion sealed to the funnel 2. A phosphor screen 4 is formed on the inner surface of the faceplate portion. The phosphor screen 4 is coated by phosphor materials of R, G, and B. A multi-apertured color selection electrode, i.e., shadow mask 3 is mounted to the screen with a predetermined space. The shadow mask 3 is hold by main and sub frames 7 and 8. An electron gun is mounted within the neck 13 to generate and direct electron beams 6 along paths through the mask to the screen.
The shadow mask 3 and the frame 7 constitutes a mask-frame assembly. The mask-frame assembly is joined to the panel 1 by means of springs 9.
The cathode ray tube further comprises an inner shield 10 for shielding the tube from external geomagnetism and a reinforcing band 12 attached to the sidewall portion of the panel 10 to prevent the cathode ray tube from being exploded by external shock. The cathode ray tube further comprises external deflection yokes 5 located in the vicinity of the funnel-to-neck junction and a magnet 11 attached to the rear side of the deflection yokes 5 for amending electron bean trajectory.
Process for making the color cathode ray tube comprises generally pre-process and post-process. During the pre-process, phosphor materials are deposited on the inner surface of the panel.
The post-process comprises further sub processes as follows. Firstly, after the phosphor materials are deposited, sealing process is performed. In the sealing process, the panel 1 to which mask-frame assembly is mounted and the funnel 2 on the inner surface of which frit is deposited is sealed together in a high temperature furnace. Then, evacuating process is performed where electron gun is inserted in the neck 13. Thereafter, an evacuating and sealing process is performed, in which the cathode ray tube is evacuated and sealed.
Since the cathode ray tube is evacuated, it suffers from high tensile and compressive stress. Therefore, a reinforcing process is conducted where reinforcing band 12 is attached to the panel to distribute the stress over the panel.
FIG. 2 shows a schematic view of distributions of stresses generated in the panel and funnel glasses after the evacuation process. As shown in FIG. 2, stress is generated by pressure difference of inside and outside of the glass envelop because the inside of the glass is evacuated. The stress changes a shape of the glass because the stress is generated at the whole glass envelop. That is, compression stress is generated at the faceplate panel and backplate funnel. Accordingly the tensile stress is generated at conor portion of the panel 1 and sealing portion of the panel and the funnel 2. In FIG. 2, dotted and solid lines represent compressive and tensile stresses, respectively.
Meanwhile, when a glass get a shock from outside, cracks appear in the glass. Tensile stress may hasten increase of the cracks such that the glass may even be broken by the cracks. On the contrary, compressive stress disturb increase of the cracks. Central portion of the panel gets compressive stress while corner portion and seal line portion get tensile stress. Therefore, the central portion is relatively strong against shock. However, the corner portion and the seal line portion is easily broken by outside shock.
Moreover, as recently cathode ray tube becomes thin on account of wide and flat screen, the stress is generated seriously. That is, the stress is generated seriously by vacuum of the glass envelop because of slimness of the cathode ray tube, maintenance of vacuum grade of the inside of the glass envelop and decrease of volumn of the cathode ray tube. Furthermore, in a case of the cathode ray tube having rectangular neck for decreasing consumption power of the deflection yoke, the cathode ray tube has constructional defect by shape of the funnel. Therefore, because high tensile stress is generated, the cathode ray tube is easily broken in heat process.
In order to solve these problems, conventional art discloses method for reinforcing the glass envelop physically by generating the compression stress through decreasing tensile stress of the glass envelop stably and performing heat process for increasing shockproof. However, in the conventional art, high tensile remain stress is generated together with the compression stress on account of ununiform temperature distribution. Therefore, because the compression stress is limited to predetermined level, to decrease weight is limited.
Furthermore, X-ray is generated when the phosphor screen is lightened by the electron beam. The X-ray which penetrates through the faceplate panel have bad influence upon human body.